This algorithm extends the patience algorithm to "support
low-occurrence common elements".

For instance, if you configured diff.algorithm variable to a
non-default value and want to use the default one, then you
have to use --diff-algorithm=default option.

--stat[=<width>[,<name-width>[,<count>]]]

Generate a diffstat. By default, as much space as necessary
will be used for the filename part, and the rest for the graph
part. Maximum width defaults to terminal width, or 80 columns
if not connected to a terminal, and can be overridden by
<width>. The width of the filename part can be limited by
giving another width <name-width> after a comma. The width
of the graph part can be limited by using
--stat-graph-width=<width> (affects all commands generating
a stat graph) or by setting diff.statGraphWidth=<width>
(does not affect git format-patch).
By giving a third parameter <count>, you can limit the
output to the first <count> lines, followed by ... if
there are more.

These parameters can also be set individually with --stat-width=<width>,
--stat-name-width=<name-width> and --stat-count=<count>.

--numstat

Similar to --stat, but shows number of added and
deleted lines in decimal notation and pathname without
abbreviation, to make it more machine friendly. For
binary files, outputs two - instead of saying
0 0.

--shortstat

Output only the last line of the --stat format containing total
number of modified files, as well as number of added and deleted
lines.

--dirstat[=<param1,param2,...>]

Output the distribution of relative amount of changes for each
sub-directory. The behavior of --dirstat can be customized by
passing it a comma separated list of parameters.
The defaults are controlled by the diff.dirstat configuration
variable (see git-config(1)).
The following parameters are available:

changes

Compute the dirstat numbers by counting the lines that have been
removed from the source, or added to the destination. This ignores
the amount of pure code movements within a file. In other words,
rearranging lines in a file is not counted as much as other changes.
This is the default behavior when no parameter is given.

lines

Compute the dirstat numbers by doing the regular line-based diff
analysis, and summing the removed/added line counts. (For binary
files, count 64-byte chunks instead, since binary files have no
natural concept of lines). This is a more expensive --dirstat
behavior than the changes behavior, but it does count rearranged
lines within a file as much as other changes. The resulting output
is consistent with what you get from the other --*stat options.

files

Compute the dirstat numbers by counting the number of files changed.
Each changed file counts equally in the dirstat analysis. This is
the computationally cheapest --dirstat behavior, since it does
not have to look at the file contents at all.

cumulative

Count changes in a child directory for the parent directory as well.
Note that when using cumulative, the sum of the percentages
reported may exceed 100%. The default (non-cumulative) behavior can
be specified with the noncumulative parameter.

<limit>

An integer parameter specifies a cut-off percent (3% by default).
Directories contributing less than this percentage of the changes
are not shown in the output.

Example: The following will count changed files, while ignoring
directories with less than 10% of the total amount of changed files,
and accumulating child directory counts in the parent directories:
--dirstat=files,10,cumulative.

--summary

Output a condensed summary of extended header information
such as creations, renames and mode changes.

--patch-with-stat

Synonym for -p --stat.

-z

When --raw, --numstat, --name-only or --name-status has been
given, do not munge pathnames and use NULs as output field terminators.

Without this option, each pathname output will have TAB, LF, double quotes,
and backslash characters replaced with \t, \n, \", and \\,
respectively, and the pathname will be enclosed in double quotes if
any of those replacements occurred.

--name-only

Show only names of changed files.

--name-status

Show only names and status of changed files. See the description
of the --diff-filter option on what the status letters mean.

--submodule[=<format>]

Specify how differences in submodules are shown. When --submodule
or --submodule=log is given, the log format is used. This format lists
the commits in the range like git-submodule(1)summary does.
Omitting the --submodule option or specifying --submodule=short,
uses the short format. This format just shows the names of the commits
at the beginning and end of the range. Can be tweaked via the
diff.submodule configuration variable.

--color[=<when>]

Show colored diff.
--color (i.e. without =<when>) is the same as --color=always.
<when> can be one of always, never, or auto.

--no-color

Turn off colored diff.
It is the same as --color=never.

--word-diff[=<mode>]

Show a word diff, using the <mode> to delimit changed words.
By default, words are delimited by whitespace; see
--word-diff-regex below. The <mode> defaults to plain, and
must be one of:

color

Highlight changed words using only colors. Implies --color.

plain

Show words as [-removed-] and {+added+}. Makes no
attempts to escape the delimiters if they appear in the input,
so the output may be ambiguous.

porcelain

Use a special line-based format intended for script
consumption. Added/removed/unchanged runs are printed in the
usual unified diff format, starting with a +/-/
character at the beginning of the line and extending to the
end of the line. Newlines in the input are represented by a
tilde ~ on a line of its own.

none

Disable word diff again.

Note that despite the name of the first mode, color is used to
highlight the changed parts in all modes if enabled.

--word-diff-regex=<regex>

Use <regex> to decide what a word is, instead of considering
runs of non-whitespace to be a word. Also implies
--word-diff unless it was already enabled.

Every non-overlapping match of the
<regex> is considered a word. Anything between these matches is
considered whitespace and ignored(!) for the purposes of finding
differences. You may want to append |[^[:space:]] to your regular
expression to make sure that it matches all non-whitespace characters.
A match that contains a newline is silently truncated(!) at the
newline.

The regex can also be set via a diff driver or configuration option, see
gitattributes(1) or git-config(1). Giving it explicitly
overrides any diff driver or configuration setting. Diff drivers
override configuration settings.

--color-words[=<regex>]

Equivalent to --word-diff=color plus (if a regex was
specified) --word-diff-regex=<regex>.

--no-renames

Turn off rename detection, even when the configuration
file gives the default to do so.

--check

Warn if changes introduce whitespace errors. What are
considered whitespace errors is controlled by core.whitespace
configuration. By default, trailing whitespaces (including
lines that solely consist of whitespaces) and a space character
that is immediately followed by a tab character inside the
initial indent of the line are considered whitespace errors.
Exits with non-zero status if problems are found. Not compatible
with --exit-code.

--full-index

Instead of the first handful of characters, show the full
pre- and post-image blob object names on the "index"
line when generating patch format output.

--binary

In addition to --full-index, output a binary diff that
can be applied with git-apply.

--abbrev[=<n>]

Instead of showing the full 40-byte hexadecimal object
name in diff-raw format output and diff-tree header
lines, show only a partial prefix. This is
independent of the --full-index option above, which controls
the diff-patch output format. Non default number of
digits can be specified with --abbrev=<n>.

-B[<n>][/<m>]

--break-rewrites[=[<n>][/<m>]]

Break complete rewrite changes into pairs of delete and
create. This serves two purposes:

It affects the way a change that amounts to a total rewrite of a file
not as a series of deletion and insertion mixed together with a very
few lines that happen to match textually as the context, but as a
single deletion of everything old followed by a single insertion of
everything new, and the number m controls this aspect of the -B
option (defaults to 60%). -B/70% specifies that less than 30% of the
original should remain in the result for Git to consider it a total
rewrite (i.e. otherwise the resulting patch will be a series of
deletion and insertion mixed together with context lines).

When used with -M, a totally-rewritten file is also considered as the
source of a rename (usually -M only considers a file that disappeared
as the source of a rename), and the number n controls this aspect of
the -B option (defaults to 50%). -B20% specifies that a change with
addition and deletion compared to 20% or more of the file's size are
eligible for being picked up as a possible source of a rename to
another file.

-M[<n>]

--find-renames[=<n>]

Detect renames.
If n is specified, it is a threshold on the similarity
index (i.e. amount of addition/deletions compared to the
file's size). For example, -M90% means Git should consider a
delete/add pair to be a rename if more than 90% of the file
hasn't changed. Without a % sign, the number is to be read as
a fraction, with a decimal point before it. I.e., -M5 becomes
0.5, and is thus the same as -M50%. Similarly, -M05 is
the same as -M5%. To limit detection to exact renames, use
-M100%.

-C[<n>]

--find-copies[=<n>]

Detect copies as well as renames. See also --find-copies-harder.
If n is specified, it has the same meaning as for -M<n>.

--find-copies-harder

For performance reasons, by default, -C option finds copies only
if the original file of the copy was modified in the same
changeset. This flag makes the command
inspect unmodified files as candidates for the source of
copy. This is a very expensive operation for large
projects, so use it with caution. Giving more than one
-C option has the same effect.

-D

--irreversible-delete

Omit the preimage for deletes, i.e. print only the header but not
the diff between the preimage and /dev/null. The resulting patch
is not meant to be applied with patch nor git apply; this is
solely for people who want to just concentrate on reviewing the
text after the change. In addition, the output obviously lack
enough information to apply such a patch in reverse, even manually,
hence the name of the option.

When used together with -B, omit also the preimage in the deletion part
of a delete/create pair.

-l<num>

The -M and -C options require O(n^2) processing time where n
is the number of potential rename/copy targets. This
option prevents rename/copy detection from running if
the number of rename/copy targets exceeds the specified
number.

--diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]

Select only files that are Added (A), Copied (C),
Deleted (D), Modified (M), Renamed (R), have their
type (i.e. regular file, symlink, submodule, ...) changed (T),
are Unmerged (U), are
Unknown (X), or have had their pairing Broken (B).
Any combination of the filter characters (including none) can be used.
When * (All-or-none) is added to the combination, all
paths are selected if there is any file that matches
other criteria in the comparison; if there is no file
that matches other criteria, nothing is selected.

-S<string>

Look for differences that introduce or remove an instance of
<string>. Note that this is different than the string simply
appearing in diff output; see the pickaxe entry in
gitdiffcore(7) for more details.

-G<regex>

Look for differences whose added or removed line matches
the given <regex>.

--pickaxe-all

When -S or -G finds a change, show all the changes in that
changeset, not just the files that contain the change
in <string>.

--pickaxe-regex

Make the <string> not a plain string but an extended POSIX
regex to match.

-O<orderfile>

Output the patch in the order specified in the
<orderfile>, which has one shell glob pattern per line.

-R

Swap two inputs; that is, show differences from index or
on-disk file to tree contents.

--relative[=<path>]

When run from a subdirectory of the project, it can be
told to exclude changes outside the directory and show
pathnames relative to it with this option. When you are
not in a subdirectory (e.g. in a bare repository), you
can name which subdirectory to make the output relative
to by giving a <path> as an argument.

-a

--text

Treat all files as text.

--ignore-space-at-eol

Ignore changes in whitespace at EOL.

-b

--ignore-space-change

Ignore changes in amount of whitespace. This ignores whitespace
at line end, and considers all other sequences of one or
more whitespace characters to be equivalent.

-w

--ignore-all-space

Ignore whitespace when comparing lines. This ignores
differences even if one line has whitespace where the other
line has none.

--inter-hunk-context=<lines>

Show the context between diff hunks, up to the specified number
of lines, thereby fusing hunks that are close to each other.

-W

--function-context

Show whole surrounding functions of changes.

--exit-code

Make the program exit with codes similar to diff(1).
That is, it exits with 1 if there were differences and
0 means no differences.

--quiet

Disable all output of the program. Implies --exit-code.

--ext-diff

Allow an external diff helper to be executed. If you set an
external diff driver with gitattributes(5), you need
to use this option with git-log(1) and friends.

--no-ext-diff

Disallow external diff drivers.

--textconv

--no-textconv

Allow (or disallow) external text conversion filters to be run
when comparing binary files. See gitattributes(5) for
details. Because textconv filters are typically a one-way
conversion, the resulting diff is suitable for human
consumption, but cannot be applied. For this reason, textconv
filters are enabled by default only for git-diff(1) and
git-log(1), but not for git-format-patch(1) or
diff plumbing commands.

--ignore-submodules[=<when>]

Ignore changes to submodules in the diff generation. <when> can be
either "none", "untracked", "dirty" or "all", which is the default
Using "none" will consider the submodule modified when it either contains
untracked or modified files or its HEAD differs from the commit recorded
in the superproject and can be used to override any settings of the
ignore option in git-config(1) or gitmodules(5). When
"untracked" is used submodules are not considered dirty when they only
contain untracked content (but they are still scanned for modified
content). Using "dirty" ignores all changes to the work tree of submodules,
only changes to the commits stored in the superproject are shown (this was
the behavior until 1.7.0). Using "all" hides all changes to submodules.

--src-prefix=<prefix>

Show the given source prefix instead of "a/".

--dst-prefix=<prefix>

Show the given destination prefix instead of "b/".

--no-prefix

Do not show any source or destination prefix.

For more detailed explanation on these common options, see also
gitdiffcore(7).
<tree-ish>::
The id of a tree object.

<path>...

If provided, the results are limited to a subset of files
matching one of these prefix strings.
i.e., file matches /^<pattern1>|<pattern2>|.../
Note that this parameter does not provide any wildcard or regexp
features.

-r

recurse into sub-trees

-t

show tree entry itself as well as subtrees. Implies -r.

--root

When --root is specified the initial commit will be shown as a big
creation event. This is equivalent to a diff against the NULL tree.

--stdin

When --stdin is specified, the command does not take
<tree-ish> arguments from the command line. Instead, it
reads lines containing either two <tree>, one <commit>, or a
list of <commit> from its standard input. (Use a single space
as separator.)

When two trees are given, it compares the first tree with the second.
When a single commit is given, it compares the commit with its
parents. The remaining commits, when given, are used as if they are
parents of the first commit.

When comparing two trees, the ID of both trees (separated by a space
and terminated by a newline) is printed before the difference. When
comparing commits, the ID of the first (or only) commit, followed by a
newline, is printed.

The following flags further affect the behavior when comparing
commits (but not trees).

-m

By default, git diff-tree --stdin does not show
differences for merge commits. With this flag, it shows
differences to that commit from all of its parents. See
also -c.

-s

By default, git diff-tree --stdin shows differences,
either in machine-readable form (without -p) or in patch
form (with -p). This output can be suppressed. It is
only useful with -v flag.

-v

This flag causes git diff-tree --stdin to also show
the commit message before the differences.

--pretty[=<format>]

--format=<format>

Pretty-print the contents of the commit logs in a given format,
where <format> can be one of oneline, short, medium,
full, fuller, email, raw and format:<string>. See
the "PRETTY FORMATS" section for some additional details for each
format. When omitted, the format defaults to medium.

Note: you can specify the default pretty format in the repository
configuration (see git-config(1)).

--abbrev-commit

Instead of showing the full 40-byte hexadecimal commit object
name, show only a partial prefix. Non default number of
digits can be specified with "--abbrev=<n>" (which also modifies
diff output, if it is displayed).

This should make "--pretty=oneline" a whole lot more readable for
people using 80-column terminals.

--no-abbrev-commit

Show the full 40-byte hexadecimal commit object name. This negates
--abbrev-commit and those options which imply it such as
"--oneline". It also overrides the log.abbrevCommit variable.

--oneline

This is a shorthand for "--pretty=oneline --abbrev-commit"
used together.

--encoding[=<encoding>]

The commit objects record the encoding used for the log message
in their encoding header; this option can be used to tell the
command to re-code the commit log message in the encoding
preferred by the user. For non plumbing commands this
defaults to UTF-8.

--notes[=<ref>]

Show the notes (see git-notes(1)) that annotate the
commit, when showing the commit log message. This is the default
for git log, git show and git whatchanged commands when
there is no --pretty, --format nor --oneline option given
on the command line.

By default, the notes shown are from the notes refs listed in the
core.notesRef and notes.displayRef variables (or corresponding
environment overrides). See git-config(1) for more details.

With an optional <ref> argument, show this notes ref instead of the
default notes ref(s). The ref is taken to be in refs/notes/ if it
is not qualified.

Multiple --notes options can be combined to control which notes are
being displayed. Examples: "--notes=foo" will show only notes from
"refs/notes/foo"; "--notes=foo --notes" will show both notes from
"refs/notes/foo" and from the default notes ref(s).

--no-notes

Do not show notes. This negates the above --notes option, by
resetting the list of notes refs from which notes are shown.
Options are parsed in the order given on the command line, so e.g.
"--notes --notes=foo --no-notes --notes=bar" will only show notes
from "refs/notes/bar".

--show-notes[=<ref>]

--[no-]standard-notes

These options are deprecated. Use the above --notes/--no-notes
options instead.

--show-signature

Check the validity of a signed commit object by passing the signature
to gpg --verify and show the output.

--no-commit-id

git diff-tree outputs a line with the commit ID when
applicable. This flag suppressed the commit ID output.

-c

This flag changes the way a merge commit is displayed
(which means it is useful only when the command is given
one <tree-ish>, or --stdin). It shows the differences
from each of the parents to the merge result simultaneously
instead of showing pairwise diff between a parent and the
result one at a time (which is what the -m option does).
Furthermore, it lists only files which were modified
from all parents.

--cc

This flag changes the way a merge commit patch is displayed,
in a similar way to the -c option. It implies the -c
and -p options and further compresses the patch output
by omitting uninteresting hunks whose the contents in the parents
have only two variants and the merge result picks one of them
without modification. When all hunks are uninteresting, the commit
itself and the commit log message is not shown, just like in any other
"empty diff" case.

--always

Show the commit itself and the commit log message even
if the diff itself is empty.

PRETTY FORMATS

If the commit is a merge, and if the pretty-format
is not oneline, email or raw, an additional line is
inserted before the Author: line. This line begins with
"Merge: " and the sha1s of ancestral commits are printed,
separated by spaces. Note that the listed commits may not
necessarily be the list of the direct parent commits if you
have limited your view of history: for example, if you are
only interested in changes related to a certain directory or
file.

There are several built-in formats, and you can define
additional formats by setting a pretty.<name>
config option to either another format name, or a
format: string, as described below (see
git-config(1)). Here are the details of the
built-in formats:

The raw format shows the entire commit exactly as
stored in the commit object. Notably, the SHA1s are
displayed in full, regardless of whether --abbrev or
--no-abbrev are used, and parents information show the
true parent commits, without taking grafts nor history
simplification into account.

format:<string>

The format:<string> format allows you to specify which information
you want to show. It works a little bit like printf format,
with the notable exception that you get a newline with %n
instead of \n.

E.g, format:"The author of %h was %an, %ar%nThe title was >>%s<<%n"
would show something like this:

The author of fe6e0ee was Junio C Hamano, 23 hours ago
The title was >>t4119: test autocomputing -p<n> for traditional diff input.<<

%C(...): color specification, as described in color.branch.* config option;
adding auto, at the beginning will emit color only when colors are
enabled for log output (by color.diff, color.ui, or --color, and
respecting the auto settings of the former if we are going to a
terminal)

%m: left, right or boundary mark

%n: newline

%%: a raw %

%x00: print a byte from a hex code

%w([<w>[,<i1>[,<i2>]]]): switch line wrapping, like the -w option of
git-shortlog(1).

Note

Some placeholders may depend on other options given to the
revision traversal engine. For example, the %g* reflog options will
insert an empty string unless we are traversing reflog entries (e.g., by
git log -g). The %d placeholder will use the "short" decoration
format if --decorate was not already provided on the command line.

If you add a + (plus sign) after % of a placeholder, a line-feed
is inserted immediately before the expansion if and only if the
placeholder expands to a non-empty string.

If you add a - (minus sign) after % of a placeholder, line-feeds that
immediately precede the expansion are deleted if and only if the
placeholder expands to an empty string.

If you add a (space) after % of a placeholder, a space
is inserted immediately before the expansion if and only if the
placeholder expands to a non-empty string.

tformat:

The tformat: format works exactly like format:, except that it
provides "terminator" semantics instead of "separator" semantics. In
other words, each commit has the message terminator character (usually a
newline) appended, rather than a separator placed between entries.
This means that the final entry of a single-line format will be properly
terminated with a new line, just as the "oneline" format does.
For example:

Limiting Output

If you're only interested in differences in a subset of files, for
example some architecture-specific files, you might do:

git diff-tree -r <tree-ish> <tree-ish> arch/ia64 include/asm-ia64

and it will only show you what changed in those two directories.

Or if you are searching for what changed in just kernel/sched.c, just do

git diff-tree -r <tree-ish> <tree-ish> kernel/sched.c

and it will ignore all differences to other files.

The pattern is always the prefix, and is matched exactly. There are no
wildcards. Even stricter, it has to match a complete path component.
I.e. "foo" does not pick up foobar.h. "foo" does match foo/bar.h
so it can be used to name subdirectories.

which tells you that the last commit changed just one file (it's from
this one:

commit 3c6f7ca19ad4043e9e72fa94106f352897e651a8
tree 5319e4d609cdd282069cc4dce33c1db559539b03
parent b4e628ea30d5ab3606119d2ea5caeab141d38df7
author Linus Torvalds <torvalds@ppc970.osdl.org> Sat Apr 9 12:02:30 2005
committer Linus Torvalds <torvalds@ppc970.osdl.org> Sat Apr 9 12:02:30 2005
Make "git-fsck-objects" print out all the root commits it finds.
Once I do the reference tracking, I'll also make it print out all the
HEAD commits it finds, which is even more interesting.

in case you care).

Raw output format

The raw output format from "git-diff-index", "git-diff-tree",
"git-diff-files" and "git diff --raw" are very similar.

These commands all compare two sets of things; what is
compared differs:

git-diff-index <tree-ish>

compares the <tree-ish> and the files on the filesystem.

git-diff-index --cached <tree-ish>

compares the <tree-ish> and the index.

git-diff-tree [-r] <tree-ish-1> <tree-ish-2> [<pattern>...]

compares the trees named by the two arguments.

git-diff-files [<pattern>...]

compares the index and the files on the filesystem.

The "git-diff-tree" command begins its output by printing the hash of
what is being compared. After that, all the commands print one output
line per changed file.

U: file is unmerged (you must complete the merge before it can
be committed)

X: "unknown" change type (most probably a bug, please report it)

Status letters C and R are always followed by a score (denoting the
percentage of similarity between the source and target of the move or
copy), and are the only ones to be so.

<sha1> is shown as all 0's if a file is new on the filesystem
and it is out of sync with the index.

Example:

:100644 100644 5be4a4...... 000000...... M file.c

When -z option is not used, TAB, LF, and backslash characters
in pathnames are represented as \t, \n, and \\,
respectively.

diff format for merges

"git-diff-tree", "git-diff-files" and "git-diff --raw"
can take -c or --cc option
to generate diff output also for merge commits. The output differs
from the format described above in the following way:

there is a colon for each parent

there are more "src" modes and "src" sha1

status is concatenated status characters for each parent

no optional "score" number

single path, only for "dst"

Example:

::100644 100644 100644 fabadb8... cc95eb0... 4866510... MM describe.c

Note that combined diff lists only files which were modified from
all parents.

Generating patches with -p

When "git-diff-index", "git-diff-tree", or "git-diff-files" are run
with a -p option, "git diff" without the --raw option, or
"git log" with the "-p" option, they
do not produce the output described above; instead they produce a
patch file. You can customize the creation of such patches via the
GIT_EXTERNAL_DIFF and the GIT_DIFF_OPTS environment variables.

What the -p option produces is slightly different from the traditional
diff format:

It is preceded with a "git diff" header that looks like this:

diff --git a/file1 b/file2

The a/ and b/ filenames are the same unless rename/copy is
involved. Especially, even for a creation or a deletion,
/dev/null is not used in place of the a/ or b/ filenames.

When rename/copy is involved, file1 and file2 show the
name of the source file of the rename/copy and the name of
the file that rename/copy produces, respectively.

File modes are printed as 6-digit octal numbers including the file type
and file permission bits.

Path names in extended headers do not include the a/ and b/ prefixes.

The similarity index is the percentage of unchanged lines, and
the dissimilarity index is the percentage of changed lines. It
is a rounded down integer, followed by a percent sign. The
similarity index value of 100% is thus reserved for two equal
files, while 100% dissimilarity means that no line from the old
file made it into the new one.

The index line includes the SHA-1 checksum before and after the change.
The <mode> is included if the file mode does not change; otherwise,
separate lines indicate the old and the new mode.

TAB, LF, double quote and backslash characters in pathnames
are represented as \t, \n, \" and \\, respectively.
If there is need for such substitution then the whole
pathname is put in double quotes.

All the file1 files in the output refer to files before the
commit, and all the file2 files refer to files after the commit.
It is incorrect to apply each change to each file sequentially. For
example, this patch will swap a and b:

diff --git a/a b/b
rename from a
rename to b
diff --git a/b b/a
rename from b
rename to a

combined diff format

Any diff-generating command can take the -c or --cc option to
produce a combined diff when showing a merge. This is the default
format when showing merges with git-diff(1) or
git-show(1). Note also that you can give the `-m' option to any
of these commands to force generation of diffs with individual parents
of a merge.

The mode <mode>,<mode>..<mode> line appears only if at least one of
the <mode> is different from the rest. Extended headers with
information about detected contents movement (renames and
copying detection) are designed to work with diff of two
<tree-ish> and are not used by combined diff format.

It is followed by two-line from-file/to-file header

--- a/file
+++ b/file

Similar to two-line header for traditional unified diff
format, /dev/null is used to signal created or deleted
files.

Chunk header format is modified to prevent people from
accidentally feeding it to patch -p1. Combined diff format
was created for review of merge commit changes, and was not
meant for apply. The change is similar to the change in the
extended index header:

@@@ <from-file-range> <from-file-range> <to-file-range> @@@

There are (number of parents + 1) @ characters in the chunk
header for combined diff format.

Unlike the traditional unified diff format, which shows two
files A and B with a single column that has - (minus --
appears in A but removed in B), + (plus -- missing in A but
added to B), or " " (space -- unchanged) prefix, this format
compares two or more files file1, file2,... with one file X, and
shows how X differs from each of fileN. One column for each of
fileN is prepended to the output line to note how X's line is
different from it.

A - character in the column N means that the line appears in
fileN but it does not appear in the result. A + character
in the column N means that the line appears in the result,
and fileN does not have that line (in other words, the line was
added, from the point of view of that parent).

In the above example output, the function signature was changed
from both files (hence two - removals from both file1 and
file2, plus ++ to mean one line that was added does not appear
in either file1 nor file2). Also eight other lines are the same
from file1 but do not appear in file2 (hence prefixed with +).

When shown by git diff-tree -c, it compares the parents of a
merge commit with the merge result (i.e. file1..fileN are the
parents). When shown by git diff-files -c, it compares the
two unresolved merge parents with the working tree file
(i.e. file1 is stage 2 aka "our version", file2 is stage 3 aka
"their version").

other diff formats

The --summary option describes newly added, deleted, renamed and
copied files. The --stat option adds diffstat(1) graph to the
output. These options can be combined with other options, such as
-p, and are meant for human consumption.

When showing a change that involves a rename or a copy, --stat output
formats the pathnames compactly by combining common prefix and suffix of
the pathnames. For example, a change that moves arch/i386/Makefile to
arch/x86/Makefile while modifying 4 lines will be shown like this:

arch/{i386 => x86}/Makefile | 4 +--

The --numstat option gives the diffstat(1) information but is designed
for easier machine consumption. An entry in --numstat output looks
like this:

1 2 README
3 1 arch/{i386 => x86}/Makefile

That is, from left to right:

the number of added lines;

a tab;

the number of deleted lines;

a tab;

pathname (possibly with rename/copy information);

a newline.

When -z output option is in effect, the output is formatted this way:

1 2 README NUL
3 1 NUL arch/i386/Makefile NUL arch/x86/Makefile NUL

That is:

the number of added lines;

a tab;

the number of deleted lines;

a tab;

a NUL (only exists if renamed/copied);

pathname in preimage;

a NUL (only exists if renamed/copied);

pathname in postimage (only exists if renamed/copied);

a NUL.

The extra NUL before the preimage path in renamed case is to allow
scripts that read the output to tell if the current record being read is
a single-path record or a rename/copy record without reading ahead.
After reading added and deleted lines, reading up to NUL would yield
the pathname, but if that is NUL, the record will show two paths.